Long description of how floats overlap with boxes in normal
flow

This diagram illustrates a floating image that overlaps the borders
of two paragraphs in normal flow: the paragraph borders are
clipped by the non-transparent parts of the image.

There are two paragraphs in normal flow, each surrounded by red
border. A left-floating image causes the final three lines of the
upper paragraph to float along its right side. It causes the
first three lines of the following paragraph to do the same, and
the remainder of that paragraph flows normally.

The paragraph borders shine through the margin area of the image
box. The non-transparent part of the image content area
clips them, however.